The present invention relates to a non-aqueous electrolytic solution and a lithium secondary battery employing the non-aqueous electrolytic solution. In particular, the invention relates to a lithium secondary battery having improved electric capacity and cycle characteristics, and a non-aqueous electrolytic solution and non-aqueous solvent which are advantageously employable for preparing the lithium secondary battery.
At present, potable small electronic devices such as personal computers, cellular phones, and video recorders equipped with camera are widely used, and a small sized secondary battery having light weight and high electric capacity is desired to provide an electric source for driving such small electronic devices. From the viewpoints of small size, light weight, and high electric capacity, a lithium secondary battery is paid attention.
The lithium secondary battery employs a positive active electrode material comprising a complex oxide such as lithium cobaltate, lithium nickelate, or lithium manganate, a negative active electrode material comprising a carbonaceous material into which lithium ions are able to intercalate and from which lithium ions are able to release, and a non-aqueous electrolytic solution of a lithium salt in a non-aqueous solvent comprising a cyclic carbonate and a linear carbonate. The lithium secondary battery is now studied for improving its characteristics.
Among the carbonaceous materials into which lithium ions are able to intercalate and from which lithium ions are able to release, graphite is considered to be the most preferred negative active electrode material of a lithium secondary battery because of its large electric capacity and advantageous flat electric potential curve, and therefore is employed widely in the art.
There is a problem, however, in that the graphite electrode shows exfoliation on its surface when it is employed in a lithium secondary battery in combination with a non-aqueous solvent for the electrolytic solution which comprises a cyclic carbonate such as ethylene carbonate (EC), propylene carbonate (PC), or butylene carbonate (BC). Simultaneously, the cyclic carbonate is decomposed on the surface of the graphite electrode. The exfoliation of the graphite electrode and decomposition of the cyclic carbonate of the non-aqueous solvent cause decrease of battery characteristics such as electric capacity, cycle characteristics, and storage stability. Particularly, the decrease is apparently observed when the graphite electrode is employed in an electrolytic solution containing propylene carbonate. It is sometimes noted that propylene carbonate decomposes on the surface of the graphite negative electrode when it is subjected to initial charging procedure and that further discharging-charging procedures cannot be done.
For obviating decomposition of an electrolytic solution on the surface of the graphite negative electrode material and exfoliation of the graphite, it has been proposed addition of additive material classified into various compounds. For instance, J. Electrochem. Soc., Vol. 140, No. 6, L 101 (1993) describes that addition of a crown-ether compound (12-crown-4) to an electrolytic solution comprising propylene carbonate and ethylene carbonate obviates decomposition of the electrolytic solution. In this case, however, it is required to use a relatively large amount of an expensive crown-ether compound for effectively obviating the decomposition. Further, the addition of crown-ether still cannot impart to the battery well satisfactory electric characteristics.
U.S. Pat. No. 5,626,981 describes an electrolytic solution comprising a lithium salt and a mixture of at least two aprotic organic solvents of which the first solvent has a high dielectric constant and the second solvent has low viscosity and further contains a soluble compound of the same type as at least one of the solvents and contains at least one unsaturated bond and which can be reduced at the anode at a potential of more than 1 volt with respect to lithium to form a passivation layer. This patent describes that the additive compound is reduced on the anode when the battery is charged, to form a passivation layer on the graphite surface and obviate reduction of other solvent components.
According to the study of the inventors, however, the methods described above cannot give satisfactorily high Coulomb efficiency (i.e., charge-discharge efficiency) at the initial stage. Further, the electric capacity gradually decreases after the charge-discharge cycle is repeated. Thus, the known improvement methods fail to impart satisfactory cycle characteristics and storage stability to the lithium secondary battery.
Further, 1997 Joint International Meeting of The Electrochemical Society, Inc. and International Society of Electrochemistry, Abstracts, P. 153 (1997) describes that a voltamograph obtained in a battery cell comprising a graphite electrode (working electrode)/Li (counter electrode)/Li (reference electrode) and an electrolytic solution of 1M LiPF6 in a solvent of PC/EC/DMC (DMC: dimethyl carbonate) of 1/1/3 by a volume ratio shows a reduction peak at 1 volt, and that the passivation film is formed on the negative electrode at that voltage so as to keep other solvent components from reducing.
Furthermore, J. Electrochem. Soc., Vol. 140, No. 9, L 161 (1995) describes that addition of chloroethylene carbonate to an electrolytic solution is effective to keep propylene carbonate (PC) from decomposing on the graphite electrode surface. It is assumed that a decomposed product of chloroethylene carbonate forms a passivation film on the graphite surface. However, the inhibition of decomposition of the electrolytic solution is not satisfactorily high.
According to the above-described improvement methods, it has become possible to use a cyclic carbonate (which is an excellent non-aqueous solvent) and a carbonaceous electrode having high crystallinity such as a graphite electrode in combination. Nevertheless, the use of the above-mentioned solvent component is still not able to provide a lithium secondary battery showing well satisfactory battery characteristics.
The present inventors have focused their studies on the use of a non-aqueous solvent mixture of a cyclic carbonate (which shows excellent characteristics as a non-aqueous solvent for an electrolytic solution) and particularly on the effect of vinylene carbonate (VC) for keeping the electrolytic solution from decomposing on the graphite electrode surface.
As a result, it has been discovered that a vinylene carbonate product prepared by a conventional synthetic process does not provide satisfactory battery characteristics, and further the resulting battery does not have reliable battery characteristics. It is further discovered that the vinylene carbonate product prepared by conventional synthetic processes contains a not small amount of chlorine atom-containing organic compounds which are produced in the process for the preparation of vinylene carbonate as by-products. The by-produced chlorine atom-containing organic compounds are incorporated into a non-aqueous solvent of an electrolytic solution when the vinylene carbonate product is mixed with other non-aqueous solvent components. The chlorine atom-containing organic compounds in the non-aqueous solvent of an electrolytic solution bring about increase of reduction potential of the non-aqueous electrolytic solution and cause lowering of the battery characteristics and reliability of the battery.
The present invention resides in a non-aqueous electrolytic solution which comprises a non-aqueous solvent comprising a cyclic carbonate, a linear carbonate and vinylene carbonate, and an electrolyte dissolved in the non-aqueous solvent, and which shows a reduction potential of less than 1 volt (or a reduction potential higher by less than 1 volt), with reference to lithium.
The invention further resides in a non-aqueous electrolytic solution which comprises a non-aqueous solvent comprising a cyclic carbonate, a linear carbonate and vinylene carbonate, and an electrolyte dissolved in the non-aqueous solvent, and which contains one or more chlorine atom-containing organic compounds in an amount of 10 ppm or less, in terms of chlorine atom content.
The invention furthermore resides in a non-aqueous solvent comprising a cyclic carbonate, a linear carbonate and vinylene carbonate, which contains one or more chlorine atom-containing organic compounds in an amount of 10 ppm or less, in terms of chlorine atom content.
The invention furthermore resides in a lithium secondary battery comprising a positive electrode, a graphite negative-electrode having a lattice spacing of 0.34 nm or less in terms of d002, and a non-aqueous electrolytic solution which comprises a non-aqueous solvent comprising a cyclic carbonate, a linear carbonate and vinylene carbonate, and an electrolyte dissolved in the non-aqueous solvent, and which shows a reduction potential of less than 1 volt, with reference to lithium.
The invention furthermore resides in a lithium secondary battery comprising a positive electrode, a graphite negative-electrode having a lattice spacing of 0.34 nm or less in terms of d002, and a non-aqueous electrolytic solution which comprises a non-aqueous solvent comprising a cyclic carbonate, a linear carbonate and vinylene carbonate, and an electrolyte dissolved in the non-aqueous solvent, and which contains one or more chlorine atom-containing organic compounds in an amount of 10 ppm or less, in terms of chlorine atom content.
The characteristic features of the invention reside specifically in the characteristics and composition of the non-aqueous electrolytic solution or the non-aqueous solvent for the electrolytic solution. Preferred are as follows:
(1) The reduction potential of the electrolytic solution is 0.9 volt or less, preferably 0.8 volt or less, more preferably in the range of 0.7 volt to 0.8 volt, with reference to lithium.
(2) The amount of chlorine atom-containing organic compounds is in an amount of 10 ppm or less, preferably 5 ppm or less, more preferably 2.5 ppm or less, in terms of chlorine atom content.
(3) The chlorine atom-containing organic compounds are incorporated into the electrolytic solution as contaminants of the vinylene carbonate.
(4) The contaminants are contained in the vinylene carbonate in an amount of not more than 100 ppm, in terms of chlorine atom content.
The mechanism of lowering of reduction potential of the non-aqueous electrolytic solution of the invention by the decrease of the chlorine atom-containing compounds as well as the mechanism of its function to improve battery characteristics of the lithium secondary battery are not clearly understood. It is assumed, however, as follows:
Vinylene carbonate (VC) products prepared by the conventionally employed synthetic methods contain at least 3,000 ppm of the below-mentioned chlorine atom-containing organic compounds: 
When the vinylene carbonate product containing such a large amount of the plural chlorine atom-containing organic compounds is incorporated into a non-aqueous solvent of the electrolytic solution in an ordinary addition amount of 1 to 10 wt. %, the chlorine atom-containing organic compounds are also incorporated into the non-aqueous solvent in an amount of approx. 30 to 300 ppm.
The chlorine atom-containing organic compounds show a reduction potential higher than vinylene carbonate and other components of the electrolytic solution, and therefore they decompose on the graphite negative electrode surface prior to the reduction of vinylene carbonate and other components, so as to form a film coverage which can keep the electrolytic solution from decompositions of vinylene carbonate and other components.
However, since thus formed film coverage on the graphite electrode surface contains chlorine therein and becomes thick, the film cannot satisfactorily keep the electrolytic solution from decomposition. In other words, the chlorine atom-containing organic compounds attached to vinylene carbonate disturb the battery characteristics-improving function of vinylene carbonate.
In consideration of the above-described discovery, the inventors have developed a process for preparing vinylene carbonate of high purity, namely, containing a markedly less amount of the chlorine atom-containing organic compound as well as a purification process.
J. Am. Chem. Soc., 75, 1263 (1953) and other publications teach that vinylene carbonate is prepared by the first step of synthesizing monochloroethylene carbonate by chlorination of ethylene carbonate (EC), and the second step of removing hydrogen chloride from the synthesized monochloroethylene carbonate in a ether solvent having a low boiling temperature utilizing an amine compound. According to the new process developed by the inventors, if the solvent employed in the second step is replaced with an ester solvent having a high boiling temperature and the vinylene carbonate product is purified by distillation or crystallization, a high purity vinylene carbonate product containing little amount of chlorine atom-containing organic compounds can be prepared.
An electrolytic solution employing thus prepared vinylene carbonate of high purity is effective to provide a lithium secondary battery showing markedly improved electric capacity, cycle characteristics and storage stability.